Aromatic polyimides have attracted a lot of interest over the last few decades, owing to their exceptionally high chemical, photochemical and thermo-oxidative stability. These unique properties make them suitable for a wide range of applications including microelectronics, aerospace, liquid crystal displays and photoelectronics. Completely aromatic polyimides, however, lack solution processibility and consequently are difficult to work with. To overcome this problem, a two stage polycondensation reaction is employed, which involves the formation of processable polyamideacids (PAA) as precursor, followed by cyclization via thermal or chemical routes to form the final insoluble polyimide. However, polyamide-acids are unstable; this inherent disadvantage limits their industrial use. This could be overcome by polyamide-ester as precursors wherein alkyl esters are incorporated, which increases the stability and provides additional synthetic flexibility. These polyamide-esters have longer shelf lives, can be resolubilized into a suitable solvent and thermally imidized. This alternative approach now allows the synthesis of otherwise inaccessible polyimide systems in high molecular weight.
Molecules with extended pi-conjugation, including pi-conjugated polymers and polymers with acene groups like perylene or naphthalene are known to display useful properties for optoelectronic applications including but not limited to photovoltaics, light emitting diodes, and field effect transistors. The extended pi-conjugation or rigidity of the molecules negatively affects the solubility. Straight or branched alkyl chains or alkyl ether chains are often attached to the pi-conjugated molecule through direct attachment or ether, ester, imide or other linkages. For example, poly(3-hexylthiophene), poly(2,5-alkoxy phenylene vinylene)s like poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene], or poly(2,5-dialkyl-1,4-phenylene)s are pi-conjugated polymers that are soluble in, and therefore solution processible from common solvents, where the base polymer without the solubilizing side chain is insoluble.
One relevant example of a pi-conjugated polymer is one that contains alkyl imide solubilizing side chains where the pi-conjugated polymer can be a polythiophene, a polyphenylene, a polynaphthalene or similar polymer where the alkyl imide is attached as a pendent group for both an electron withdrawing effect and for an increase in solubility. In the case of a polythiophene backbone, the alkylimide pendent material has shown excellent properties as a donor material in photovoltaics with the group named as thieno[3,4-c]pyrrole-4,6-dione (TPD). The acene groups like perylene and naphthalene are also pi-conjugated molecules that display reduced solubility especially when incorporated into larger molecules. The solubility of perylene in particular has often been improved by attaching alkyl chains to various positions on the ring structure or as alkyl imide chains from the perylene dianhydride. Perylene and naphthalene dianhydrides are difunctional molecules that can be reacted through their anhydride groups into diimides or polyimides, however the solubility of the resulting diimides or polyimides is severely minimized if reacted with other rigid aromatic molecules.
Based on the imide structure of the polyimides, they can be broadly classified into two classes: five- and six-membered. Polymers with six-membered imide rings such as polynaphthalinimides (PNIs) and polyperyleneimides (PPIs) are thermally and chemically more stable compared to the five-membered phthalic polyimides. PPIs have recently emerged as a new class of n-type polymers for application in polymer solar cells. 3,4,9,10-perylene tetracarboxylic dianhydride (PTDA) and 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA) are two commonly found starting materials from which six-membered polyimide could be obtained. The extended pi-conjugation found in these dianhydrides makes them good electron acceptors and conductors. Their planar structure makes it possible to synthesize well ordered thin films on various substrates increasing luminescence and charge transfer properties. However, due to the rigid planar nature of the molecule, incorporating it into a polymer has been rather difficult and, consequently, less studied. Bisimides and polyimides incorporating perylene and naphthalene units for better performance in opto-electronics have been reported in the literature. Similarly, perylene based polyimides with different alkyl chain lengths ranging from C3 to C12 and their structural characteristics have been reported. It has often been observed that synthesis of perylene and naphthalene moiety containing polyimides that are solution processable in their polyamide-ester form are highly desirable for incorporation in optoelectronic materials.
Rigid molecules like pi-conjugated polymers and polyimides often suffer from a decreased solubility compared to more flexible molecules. Solubilization of rigid molecules is important because the molecules often need to be dissolved in a solvent for their synthesis, and the processing of molecules from solution is important for the preparation of films and fibers. Two methods have been shown to be effective for the solution synthesis and/or processing of rigid molecules. The first method is the attachment of flexible alkyl chains to increase the solubility of rigid molecules. The second method is to prepare a soluble precursor molecule that upon thermal or other reaction results in the formation of the rigid and less soluble molecule. Each method has advantages and drawbacks with numerous examples that can be found in the literature. Alkyl side chains can promote solubility but also introduce problems in efficient packing or ultimate thermal stability in the solid state. Soluble precursor molecules can undergo unwanted side reactions or incomplete reaction upon thermal conversions to rigid molecules.